High melt strength polypropylene has long been used in various extrusion processes for the production of fibers, cast and blown film, pipe and tube extrusion, blow molding and sheet extrusion. In the area of extrusion coating, although traditionally dominated by autoclave LDPE products, where LDPE is blended with propylene homopolymers to improve melt strength, high melt strength polypropylene has also been used where specific properties are required, such as in non-woven coatings, retortable bricks and metal coating. High melt strength polypropylene has also been used as a blend component with other polypropylenes in extrusion coating, because like LDPE, it possesses a long chain branched structure which is resistant to the elongational shear stress of flat die extrusion. For example, U.S. Pat. No. 5,508,318 discloses compounded blends of irradiated and non-irradiated olefin polymer materials suitable for extrusion coating applications requiring low gloss.
Techniques to improve melt strength in polypropylene have included irradiation of conventional flake polypropylene in reduced-oxygen environments, as described, for example, in U.S. Pat. Nos. 4,916,198, 5,047,485, 5,414,027, 5,541,236, 5,554,668, 5,591,785, 5,731,362, and 5,804,304. These irradiation methods increase propylene polymer melt strength by creating polymer radicals during irradiation which then re-combine to form long-chain branches in the reduced oxygen environment. High melt strength products produced by such methods, e.g., Pro-fax® PF814 and Pro-fax® PF611, have been successfully used commercially.
Conventionally, phenolic antioxidants have long been used to improve polymer stability under elevated temperature conditions, such as those typically experienced during extrusion, or during extended periods of storage. However, their use in irradiated compositions undermines enhanced melt strength by scavenging free radicals, thereby reducing the number of polymeric free radicals available to recombine to form long-chain branches. Moreover, irradiation of phenolic antioxidant-containing polymers can result in the formation of degradation products that impart undesirable color. Non-phenolic stabilizers have been used in the irradiation of conventional polyolefin materials to avoid such problems, as described in U.S. Pat. No. 6,664,317. International Publication No. WO 2009/003930 discloses irradiation of high melt strength polypropylene in the form of pellets containing non-phenolic antioxidants. However, a challenge facing the production of mixtures of conventionally produced high melt strength, where polypropylene flake is irradiated, and other polypropylenes has been that their production requires multiple melt extrusion steps using specialized extruders to homogenize the distribution of high molecular weight material in the overall composition. Moreover, altering the blend ratio of a high melt strength component produced by irradiation and a non-irradiated component to accommodate changing product needs of customers typically requires time consuming reformulation and compounding of the new blend at the polymer manufacturing site. Therefore, for extrusion processes utilizing high melt strength material containing both irradiated and non-irradiated materials, an ongoing need exists for a process that increases manufacturing flexibility, while also maintaining desired product properties. Accordingly, it has unexpectedly been found that extrusion processes using blended mixtures of an irradiated extrudate of polypropylene containing a non-phenolic stabilizer, and a non-irradiated polypropylene, blended below their melting points, provide good product performance while adding flexibility to the manufacturing process.